1. Field of the Invention
The present invention relates to SMIF pods, and more particularly to systems for allowing gas to be controllably injected into and/or removed from the pods.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (.mu.m) to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half .mu.m and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
In practice, a SMIF pod is set down on various support surfaces within a wafer fab, such as for example at a load port to a minienvironment, whereupon interface mechanisms in the load port open the pod door to allow access to the wafers within the pod. Additionally, a pod may be supported at a storage location while awaiting processing at a particular tool. Such storage locations may comprise a local tool buffer in the case of metrology or high throughput tools, or may alternatively comprise a stocker for storing large numbers of pods within a tool bay. A pod may additionally be positioned at a stand-alone purge station.
Whether a tool load port, local tool buffer, stocker or purge station, the support surfaces typically include registration or kinematic pins protruding upward from the support surface. In 200 mm pods, the support surface includes registration pins and guide rails which guide the pod into the proper rotational and translational position with respect to the pins. In 300 mm pods, a bottom surface of the pods includes radially extending grooves for receiving kinematic pins. Once the pod is positioned so that the grooves engage their respective kinematic pins, the grooves settle over the pins to establish six points of contact between the pod and support platform (at the grooves and pins) to kinematically couple the pod to the support platform with fixed and repeatable accuracy. Such a kinematic coupling is for example disclosed in U.S. Pat. No. 5,683,118, entitled "Kinematic Coupling Fluid Couplings and Method", to Slocum, which patent is incorporated by reference herein in its entirety. The size and location of the kinematic pins are standardized so that the pods of various suppliers are compatible with each other. The industry standard for the location and dimensions of the kinematic coupling pins are set by Semiconductor Equipment and Materials International ("SEMI").
Occasionally, it is advantageous to purge a pod of contaminants and/or particulates by creating a current flow through a pod to carry away the contaminants and/or particulates. It may also be beneficial to fill a pod with a non-reactive gas for longer term storage and certain processes. Additionally, it may be advantageous on occasion to provide the pod with a pressure higher or lower than ambient. In order to accomplish such purging, it is known to provide one or more valves within a pod which allow fluid flow to and/or from the interior of the pod. Inlet valves to the pod may be connected to a pressurized gas source to fill the pod with a desired gas, and outlet valves may be connected to a vacuum source to remove gas from the pod. The inlet and outlet valves may be used to purge the pod, including filling the pod with a desired gas, and/or providing a pressure differential within the pod relative to ambient. Such a system is disclosed in U.S. Pat. No. 4,724,874, entitled "Sealable Transportable Container Having a Particle Filtering System", to Parikh et al., which patent is assigned to the owner of the present application, and which patent is hereby incorporated by reference in its entirety. Relative to systems which require opening of the pod for purging, valve systems require less components and space, and in general operate more efficiently.
An interface seal in the form of an elastic member is typically provided between the gas flow pin and valve to ensure a tight fit of the flow pin with respect to the valve. It is important that such interface seals provide a tight fit, and be durable to ensure that the tight fit does not deteriorate with use. A tight seal between the valve and flow line is generally more important at the inlet valve as opposed to the outlet valve. It is important to provide a uniform and controlled purge flow through the pod from the gas injected through the inlet valve. A tight seal is a significant factor in controlling the purge flow. Additionally, the gas injected into the pod, typically nitrogen, can be harmful to fab personnel if released in large quantities, and it is important to provide a tight seal to prevent significant leakage of the gas around the seal and into the fab. It is therefore desirable to provide an effective interface seal at the inlet valve between the pod and the support surface once the pod is loaded onto the support surface.
The outlet holes are connected to a low pressure source and typically include a loose seal, referred to as a proximity seal. Gas which is forced out of the pressurized pod through the outlet is drawn away by the low pressure source so that it does not escape to ambient. To the extent that the low pressure source attempts to pull a greater volume of gas than is escaping from the pod, the proximity seal allows ambient air to be drawn around the seal, thus ensuring that the negative pressure source does not create a negative pressure within the pod.
In conventional valve systems, the interface seal is established in general as a result of a weight of pod downward, and a force of interface seal and pressurized gas upward. The interface seal has to be relatively rigid to exert the necessary counterforce upward against the weight of the pod. However, the problem with such conventional interfaces is that the downward force of the weight of the pod has to be precisely matched to the upward force of the rigid interface in response to the weight of the pod. Where the upward force of the interface is not enough to match the downward force of the pod, the pod seats on the kinematic pins without establishing a tight seal at the interface. On the other hand, where the upward force of the interface is large relative to the downward force of the pod, the interface seal interferes with a clean seating of the pod on the kinematic pins. Moreover, the weight of the pod on the seal tends to wear down the rigidity of the seal over time. Further still, the flow rate of the incoming gas may vary, thus varying the upward force of the gas against the pod. These factors further complicate the problem of matching the upward and downward forces of the pod and interface seal.
Another problem with current valve interface systems relates to the angle at which a pod is loaded onto the support surface. As explained above, in 200 mm SMIF systems, the support surface includes guide rails which position and orient the pod with respect to the registration pins. Thus, the registration holes in 200 mm pods are positioned directly over the pins before the pod is lowered down onto the pins. In 200 mm SMIF systems therefore, the valve on the bottom of 200 mm pods is similarly brought straight down on top of the gas flow pins, and there is no danger of the pod bottom catching on the gas flow pins and/or interface seal, which catching may otherwise occur if the pod were brought down on the support surface from an angled approach.
However, it is a feature of 300 mm pods that the kinematic coupling may be easily established substantially regardless of the approach angle of the pod with respect to the support surface. Once the pod has settled onto the kinematic pins in a six-point contact, the pod is in a fixed, precise and repeatable position on the support surface. However, due in part to the flexibility of the approach angle, when a pod initially engages the support surface, the flat bottom of the pod or one of the sloped surfaces of the grooves may engage any of the pins before the six-point contact is established. Thus, after initial contact, the pod may be positioned at a wide variety of angles with respect to the support surface. It is from these wide variety of initial angles that the pod settles over the pins, and the valves settle over the inlet and outlet valves in the desired positions.
In conventional purge systems, the gas flow lines from the positive pressure source terminate at the support surface at hollow flow pins that protrude above the support surface, which pins are received in the inlet valves when the pod is seated on the kinematic pins. However, as explained above, the initial engagement angle of a 300 mm pod with respect to the support surface may vary significantly, anywhere from approximately 0.degree. to 90.degree., and conventional flow pins and/or interface seals may catch on the pod at lower engagement angles. This may interfere with the kinematic coupling, and cause wear to the seal over time.
In an attempt to solve the problem of pod loading, it is known to provide "active" gas flow pins within the support surface. Active pins are those which are initially retracted below the support surface to allow the pod to be loaded onto the support surface from any angle. Thereafter, the gas flow pins may be raised upward into engagement with the gas flow valves. Such active systems require additional components and require additional controls to ensure that the components are raised and lowered in the proper fashion and at the proper time. Additionally, such active systems disadvantageously exert upward forces on the pod which may interfere with the kinematic coupling as described above.